Oxygen is a gas having a great interest industrially because it has multiple applications in numerous technical fields, for example in the production of steel, glass or paper, medicine, metal welding, combustion or depollution.
One of the techniques used at present to produce oxygen is the so-called "PSA" technique (Pressure Swing Adsorption); there is meant by PSA processes, not only PSA processes properly so-called, but also analogous processes, such as VSA (Vacuum Swing Adsorption) processes or MPSA (Mixed Pressure Swing Adsorption) processes.
Thus, it is usual to separate oxygen from a gaseous mixture comprising essentially oxygen and nitrogen, such as air, by adsorption of the nitrogen on a material preferentially adsorbing nitrogen, said adsorption of nitrogen being carried out by variation of the pressure applied in the separation zone and containing said adsorbent material; oxygen that does not adsorb or only a little absorbs, will be recovered at the outlet of said separation zone. Such PSA processes are conventionally described in the prior art.
Schematically, a PSA process for the separation of oxygen from a gaseous mixture containing essentially oxygen and nitrogen, such as air, further comprises:
a step of selective adsorption of the nitrogen on an adsorbent material and at an adsorbent pressure called "high pressure"; PA1 a step of desorption of the nitrogen trapped by the adsorbent, at a desorption pressure, called "low pressure", lower than the adsorption pressure; PA1 a step of repressurization of the adsorption zone comprising the adsorbent, from the low pressure to the high pressure; PA1 an increase of adsorption per unit volume of the particles, by, in particular, a better rearrangement of the particles among themselves, and hence a decrease of the porosity of the bed of adsorbent balls, PA1 an increase in the kinetics of the molecular sieve, PA1 and an increase in pressure drop. PA1 the particles of adsorbent material are particles of zeolite X or LSX (for Low Silica X) having a Si/Al ratio comprised between about 1 and about 1.5, preferably of the order of 1; PA1 the particles of adsorbent material are particles of exchanged zeolite; PA1 the thickness of the particle bed is comprised between 0.1 and 3 m, preferably between 0.2 and 2 m and, still more preferably between 0.3 and 1.2 m. PA1 the high adsorption pressure is comprised within the range 10.sup.5 Pa to 10.sup.6 Pa; and/or the low pressure of desorption is comprised in the range 10.sup.4 Pa to 10.sup.5 Pa; PA1 said particles having a mean size comprised between 0.3 and 3 mm, preferably between 0.5 and 2.2 mm, preferably between 0.6 and 1.6, preferably between 0.8 and 1.2 mm; PA1 the zeolite particles contain cations selected from the group consisting of calcium, lithium, zinc, strontium, magnesium, copper, aluminum, nickel, cobalt, manganese, chromium, barium, sodium, scandium, gallium, iron, indium, yttrium, lanthanides and their mixtures, PA1 the particles of adsorbent are particles of zeolite X or LSX exchanged to the extent of at least 60%, preferably at least 70%, still preferably at least 80%, with cations of lithium and/or calcium; PA1 the gaseous feed is at a temperature comprised between 15.degree. C. and 55.degree. C., preferably 20.degree. to 45.degree. C.; PA1 The first component is nitrogen and/or the second component is oxygen; preferably, the gaseous feed is air.
oxygen or a gas enriched in oxygen being recovered during the adsorption phase of the nitrogen.
The efficiency of separation of the gaseous mixture, and hence of recovery of oxygen, depends on numerous parameters, namely, particularly the high pressure, the low pressure, the type of adsorbent material and its affinity for the components to be separated, the composition of the gaseous mixture to be separated, the temperature of said mixture to be separated, the granulometry, which is to say the size and shape of the adsorbent particles used, the composition of these balls, the temperature gradient prevailing within the adsorbent bed, the geometry of the adsorbers . . . .
Until now, no law of general behavior has been until now worked out, the publications which can be found in the prior art dealing in general only with one of these parameters, for example the type of adsorbent used in the separation process, the adsorption and desorption pressures, the temperature of the air to be separated . . . .
Certain papers nevertheless deal more specifically with the granulometry, which is to say the mean size and shape of the adsorbent particles, in general "balls" of zeolite, used in the PSA process.
Thus, it is known that the granulometry of the particles plays a role in the efficiency of separation of the constituents of gaseous mixture, such as the separation of the constituents nitrogen and oxygen from air.
Thus, the balls of adsorbent of small diameter or "small balls", for example zeolite balls of less than 2.5 mm diameter, are more effective than balls of larger size because they permit decreasing the cycle time of the PSA process and, correspondingly, produce more oxygen for a given time. However, small balls have several drawbacks tending to detract from the good operation of the PSA process. In particular, they give rise to higher pressure drop within the adsorbent bed and, because of their small diameter, they are susceptible to pass easily through grills of adsorbers designed to retain them.
Conversely, although large balls give rise to less pressure drop within the adsorbent bed, pass less easily through grills of the adsorbers and have higher resistance, particularly to crushing, their use in PSA processes generally leads to a separation that is less effective and rarely optimum, of the constituents of the gaseous mixture to be separated.
There exist in the prior art publications describing the ranges of sizes of balls of zeolite, which is to say the ranges of mean granulometry of the adsorbent particles, in general less than 5 mm.
Thus, the documents EP-A-8619, U.S. Pat. No. 4,194,892 and EP-A-0488926 disclose processes of the RPSA (Rapid Pressure Swing Adsorption) type, using balls whose size is comprised respectively between 0.125 mm and 0.84 mm, between 0.12 and 0.85 mm and between 0.05 and 0.20 mm. It must nevertheless be pointed out that the RPSA processes operate opposite the PSA processes, which is to say that in an RPSA process, it is sought to establish a large pressure drop.
Moreover, the documents U.S. Pat. No. 5,174,979 and U.S. Pat. No. 4,544,378 disclose gas separation processes using zeolites having a mean granulometry comprised between 8 and 12 mesh (namely 1.65 to 2.36 mm).
The document U.S. Pat. No. 4,925,460 teaches a gas separation process using balls of zeolite having a size at least 0.12 mm.
Schematically, all the previous documents describe ranges of mean ball size, which is to say ranges of mean granulometry adapted to promote the use of the PSA process.